1. Field of the Invention
The invention relates to an electric motor drive system for an electric vehicle, and more particularly, a system that drives and controls an alternating-current electric motor by converting direct-current voltage that has been variably controlled by a converter into alternating-current voltage by an inverter.
2. Description of the Related Art
A system that drives and controls a three-phase alternating-current electric motor by converting direct-current voltage (DC voltage) into alternating-current voltage (AC voltage) with an inverter is in general use. With this kind of electric motor drive system, the motor current is typically controlled according to pulse width modulation (PWM) control based on vector control in order to drive the electric motor efficiently. Japanese Patent Application Publication No. 2006-311768 (JP-A-2006-311768) and Japanese Patent Application Publication No. 2007-325351 (JP-A-2007-325351), for example, describe structures in which the output voltage of a power storage device (represented by a secondary battery) is variably controlled and converted to direct-current side voltage for an inverter, by providing a converter that performs direct-current voltage conversion.
With this kind of an electric motor drive system, determining how to set the controllable output voltage of the converter, i.e., the input side voltage of the inverter, becomes an issue. For example, JP-A-2006-311768 describes a system that sets a voltage command value so that the modulation factor in PWM control matches a modulation factor target value set taking system loss into account. Also, JP-A-2007-325351 describes a system that sets a voltage command value so as to minimize power loss in the overall system that includes a battery, a converter, an inverter, and a motor-generator, according to the state of the motor at that time.
Also, similarly, in an electric motor drive system provided with a converter, JP-A-2006-311768, JP-A-2007-325351, and Japanese Patent Application Publication No. 2009-95144 (JP-A-2009-95144) describe using control modes other than sine wave PWM control. These publications describe using sine wave PWM control for reducing torque fluctuation in a low speed region, while applying an overmodulation PWM control mode in a medium speed region, and rectangular wave voltage control in a high speed region. Also, International Publication (Republication) No. WO2005/088822, for example, describes expanding the modulation factor range where sine wave PWM control can be applied by superimposing a harmonic wave.
However, if a control structure is used that changes the converter output voltage frequently in order to match the actual modulation factor with the modulation factor target value, as described in JP-A-2006-311768, drivability of the electric vehicle that is run by the output of the electric motor may decrease due to reduced controllability of the output torque of the electric motor. Alternatively, power loss may end up increasing due to the voltage conversion operation of the converter changing frequently.
On the other hand, setting the system voltage focusing only on the overall loss, as described in JP-A-2007-325351, is advantageous in that rectangular wave voltage control in which there are relatively few switches in the inverter is applied. However, with rectangular wave voltage control, the torque controllability of the electric motor is lower than it is with sine wave PWM control, so if priority is given only to reducing system loss, i.e., improving fuel efficiency, drivability of the electric vehicle may decrease due to reduced torque response.